In today’s charging world of technology there have been may changes in controls that now allow to truly compare a plug and seat regulator to a control valve for high-pressure natural gas installations such as: power plants, city gate stations, large industrial customers, compressor stations, and storage fields. The features, benefits, capabilities, and differences of both devices will be outlined, to enable the reader to make an educated selection. In addition, acceptable design practices will be reviewed concerning sizing, gas velocities, noise levels, equipment layout, and performance. The ball valve is the most commonly used type of modulating valve for natural gas pipeline control applications, for that reason, we will limit this discussion to comparison between the plug and seat regulator and versions of a 1/4 turn ball valve.
The basic information needed to size the control device includes: inlet pressure (min/max), total load (min/max), outlet pressure (min/max), inlet and outlet pipe size, any noise level limitations, desired failure mode, and level of shut-off requirements. It is also important to know the limits of the device you are considering for the application; the manufacturer may have an optimum range of control, such as, 15 – 85% of the capacity of the device. This information may not be part of the sizing program.
In each application, the data will be evaluated, and a determination made as to the type of device that will be used to accomplish your desired result. If they do not have an engineer on staff, they may elect to retain an engineering firm to design their stations and recommend the equipment to be utilized. The concept of pressure regulation involves the movement of gas and movement creates velocity. Gas moving through a pipeline at 100 ft/sec is moving at approximately 68 mph – and at 200 ft/sec approximately 136mph. A good rule of thumb is that when inlet pressure is more than twice the outlet pressure (both absolute), critical conditions will exist, and high velocity will be present. Elbows, tees, flow restricting valves, reducers, and increasers will create turbulence affecting velocity and capacity.